1. Field of the Invention
The present invention relates, in general, to a filter circuit and, more particularly, to a filter circuit having a transconductance amplifier.
2. Description of the Related Art
The conventional filter circuit is disclosed in xe2x80x9cA circuit design method of an analog filterxe2x80x9d, page 146 lines 11-17 and FIG. 4.57, Toshio Horikawa, SOGODENSHISYUPANSYA, and Japanese Patent Application Laid Open Number 11-112286.
Horikawa discloses, as shown in FIGS. 6(A)-6(C), an LC ladder type filter circuit which has three types. One type, as shown in FIG. 6(A), is that a resistance is supplied between two input ports and between two output ports, respectively (R-R type). Another type, as shown in FIG. 6(B), is that a resistance is only supplied between two input ports (R-∞ type). The other type, as shown in FIG. 6(C), is that a resistance is only supplied between two output ports (0-R type).
FIG. 7 is a circuit diagram showing a differential input type Low Pass Filter (LPF) circuit which is R-R type. In FIG. 7, the LPF circuit has an input resistance portion 710, a filter portion 720 and an output end resistance portion 730. The input resistance portion 710 comprises two transconductance amplifiers 711 and 712.
However, the input dynamic range of the LPF circuit depends on the input dynamic range of the transconductance amplifier 711. The voltage amplitude of the differential input signal to be input to the LPF circuit, is a value that one transconductance amplifier shares.
FIG. 8 is wave diagram showing frequency characteristics of the differential input type LPF circuit. In FIG. 8, the vertical axis shows gain and the horizontal axis shows frequency. The cutoff frequency of the differential input type LPF circuit is shown as xe2x80x9cfcxe2x80x9d. The left side of fc is a passing band and the right side of fc is a cutoff band. When a mixed signal having frequency f1 which is in the passing band and frequency f2 which is in the cutoff band is input to the differential input type LPF circuit, a voltage amplitude peak of the mixed signal is the sum of voltage amplitude of the frequency f1 and the frequency f2. When the input dynamic range of the transconductance amplif1er 711 is Vdr and when the voltage amplitude peak of the mixed signal to be input is over Vdr, the operation of the differential input type LPF circuit is not guaranteed.
According to one aspect of the present invention, there is provided a filter circuit of an input resistance portion, wherein said input resistance portion includes a first transconductance means having a positive input port, a negative input port, a positive output port and a negative output port, wherein the positive input port is impressed with a first voltage, and wherein the negative input port is impressed with a second voltage; a second transconductance means having a positive input port, a negative input port, a positive output port and a negative output port, wherein the positive input port is impressed with the second voltage, and wherein the negative input port is impressed with a third voltage; and a third transconductance means having a positive input port, a negative input port, a positive output port and a negative output port, wherein the positive input port is coupled to the positive output ports of the first and second transconductance means and the negative output port of the third transconductance means, and wherein the negative input port is coupled to the negative output ports of the first and second transconductance means and the positive output port of the third transconductance means.